C.F. Ng

Magnesia-carbon nanotubes (MgO-CNTs) nanocomposite: novel support of Ru catalyst for the generation of COx-free hydrogen from ammonia

Magnesia–carbon nanotubes (abbreviated as MgO–CNTs) nanocomposites were prepared by impregnation of CNTs with Mg(NO3)2·6H2O in ethanol solution, followed by drying at 353 K and calcination at 873 K, respectively. The nanocomposites are thermally more stable than CNTs in a H2 flow. The use of the nanocomposites as support yielded more efficient Ru catalysts for the generation of COx-free hydrogen from NH3 decomposition.

METHANE DISSOCIATION ON NI, PD, PT AND CU METAL (111) SURFACES : A THEORETICAL COMPARATIVE STUDY

Abstract A theoretical comparative study of methane dissociation on Ni, Pd, Pt and Cu metal (111) surfaces has been carried out using a quasirelativistic density functional method. Reaction energies for the steps involved in the dissociation of methane are determined. The activation energies have been estimated using the analytic BOC-MP formula. The results support the notion that the transition metals are active in methane dissociation. The calculated total dissociation energies for the complete dissociation of CH 4 to surface C and H on the transition metals fall in the order Ni Pd ≈ Pt). The complete dissociation on Cu is calculated to be endothermic. Thus methane dissociation on a Cu catalyst is unlikely, in agreement with the experimental observations. The dissociation of methane in the presence of adsorbed oxygen has also been examined.

The catalytic performance and characterization of a durable perovskite-type chloro-oxide SrFeO3–δClσ catalyst selective for the oxidative dehydrogenation of ethane

The catalytic performances and properties of SrFeO3-0.190 and SrFeO3-0.382Cl0.443 catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE). XRD results showed that both catalysts exhibited oxygen-deficient perovskite-type structures. The inclusion of chloride ions in the SrFeO3-δlattice matrix can significantly enhance ethene selectivity and ethane conversion. The SrFeO3-0.382Cl0.443 catalyst showed an ethane conversion of ca. 90%, an ethene selectivity of ca. 70%, and an ethene yield of ca. 63% under the reaction conditions: C2H6:O2:N2 = 2:1:3.7, temperature 680°C, and space velocity 6000 ml h-1 g-1. With the increase of space velocity, ethane conversion decreased, whereas ethene selectivity increased over SrFeO3-0.382Cl0.443. Lifetime studies showed that the perovskite-type chloro-oxide catalyst was durable. The results of O2-TPD and TPR experiments illustrated that the implanted chloride ions caused the oxygen nature of SrFeO3-δ to change. By regulating the concentration of oxygen vacancies and the Fe4+/Fe ratio in this perovskite-type chloro-oxide catalyst, one can generate a durable chloro-oxide catalyst for the ODE reaction with excellent performance.

Catalytic Production of Carbon Nanotubes by Decomposition of CH4 over the Pre-reduced Catalysts LaNiO3, La4Ni3O10, La3Ni2O7 and La2NiO4

A large amount of more graphitic carbon nanotubes with a narrow size distribution was produced from catalytic decomposition of CH4 over pre-reduced LaNiO3, La4Ni3O10, La3Ni2O7 and La2NiO4. The structure and component of fresh and reduced LaNiO3, La4Ni3O10, La3Ni2O7 and La2NiO4 were determined by X-ray diffraction (XRD). The carbon nanotubes obtained were characterized by means of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Thermal oxidation of carbon nanotubes in air was made by thermogravimetric experiments (TG). The results revealed that the value of La/Ni in different catalyst precursors influences the diameter distribution and graphitic degree of carbon nanotubes. Lower La/Ni leads to wider diameter and higher graphitic degree of carbon nanotubes.

The oxidative coupling of methane overBa/CO3LaOCl catalysts

Abstract The performance of LaOCl at 800°C was promoted by BaCO 3 in OCM reaction. When 10 mol% BaCO 3 was added, there was little change in CH 4 conversion, but C 2 selectivity was increased from 37% to 66%. With the increase in BaCO 3 loading, the Ba/CO 3 LaOCl catalysts decreased in specific surface area. The improvement in C 2 selectivity is partly due to surface area diminution. The addition of BaCO 3 has also caused surface modification of LaOCl. In the absence of BaCO 3 , LaOCl in OCM reaction is capable of generating dioxygen species O 2 2− , O 2 n− and O 2 − , which can cause deep oxidation of CH 4 , C 2 H 6 and C 2 H 4 . With the addition of BaCO 3 , sites for the activation of oxygen molecules are depleted. The nearby Cl − ions have the ability of destabilizing the BaCO 3 , causing it to decompose at ca. 780°C, a decomposition temperature about 200°C lower than that of pure BaCO 3 . With the deprivation of dioxygen species, the CH 4 oxidative dehydrogenation process is enhanced, while deep oxidation processes are suppressed. Compared with the deep oxidation reaction processes, the number of sites required in the H-abstraction process is a lot fewer. Hence, although the specific surface area of LaOCl was reduced with the addition of BaCO 3 , the conversion of CH 4 over the Ba/CO 3 LaOCl catalysts did not drop significantly.

Mechanistic studies of CO2/CH4 reforming over Ni–La2O2/5A

The mechanism of CO2/CH4 reforming over Ni–La2O3/5A has been studied. The results of the CO2‐pulsing experiments indicated that the amount of CO2 converted was roughly proportional to the amount of H present on the catalyst, implying that CO2 activation could be H‐assisted. Pulsing CH4 onto a H2‐reduced sample and a similar sample pretreated with CO2, we found that CH4 conversion was higher in the latter case. Hence, the idea of oxygen‐assisted CH4 dissociation is plausible. The fact that the amount of CO produced in 10 pulses of CO2/CH4 was larger than that produced in 5 pulses of CO2 followed by 5 pulses of CH4, indicated that CO2 and CH4 could activate each other synergistically. In the chemical trapping experiments, following the introduction of CD3I onto a Ni–La2O3/5A sample pretreated with CH4/CO2, we observed CD3COOH, CD3CHO, and CD3OCD3. In the in situ DRIFT experiments, IR bands attributable to formate and formyl were observed under working conditions. These results indicate that formate and formyl are intermediates for syngas generation in CO2/CH4 reforming, and active O is generated in the breaking of a C–O bond. Based on these results, we suggest that during CO2/CH4 reforming, CO2 activation is H‐promoted and surface O species generated in CO2 dissociation reacts with CHx to give CO. A reaction scheme has been proposed.

OXIDATIVE COUPLING OF METHANE OVER LAF3/LA2O3 CATALYSTS

We studied the oxidative coupling of methane over the LaF3/La2O3 (50∶50) catalyst. The catalyst was found active even at 873 K. At 1023 K, the C2 yield was 12.7% at 26.0% CH4 conversion and 49.1% C2 selectivity. It was found to be stable and had a lifetime not less than 50 h at 1023 K. The catalyst was effective in C2H6 conversion to C2H4. XRD results indicated that the catalyst was mainly rhombohedral LaOF. It is suggested that the catalyst has ample stoichiometric defects and generates active oxygen sites suitable for methane dehydrogenation.

Characterization of BaX2/Gd2O3 (X=F, Cl, Br) catalysts for the oxidative coupling of methane

Abstract The reactivities of the BaX2/Gd2O3 (X=F, Cl, Br) catalysts in the oxidative coupling of methane (OCM) reaction have been investigated. It has been observed that BaF2, BaCl2, or BaBr2 can significantly improve the catalytic performance of Gd2O3. The X-ray diffraction (XRD) results indicated that ionic substitutions between the BaX2 and Gd2O3 phases occurred during the preparation of the BaX2/Gd2O3 catalysts as well as in the course of the OCM reaction. O2-TPD, O2-pulse, and Raman results showed that such ionic substitutions could lead to the increase in the amount of oxygen adsorbed on the BaX2/Gd2O3 catalysts, resulting in the promotion of catalytic performance. Based on the results of XPS and halide content analyses, we realized that the halides in the BaX2/Gd2O3 catalysts behaved rather differently in the OCM reaction. The catalytic performance of the BaX2/Gd2O3 catalysts as related to reaction time can be correlated to the behaviours of the halides in the catalysts.

Halogenated La1.6Sr.04CuO4 catalysts active for ethane selective oxidation to ethene

AbstractThe catalytic performances and characterization of the catalysts La1.6Sr0.4CuO3.852, La1.6Sr0.4CuO3.857F0.143, and La1.6Sr0.4 CuO3.856Cl0.126 have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. X‐ray diffraction results indicated that the three catalysts have a single‐phase tetragonal K2NiF4-type structure. The incorporation of fluoride or chloride ions in the La1.6Sr0.4CuO4-δ lattice can significantly enhance C2H6 conversion and C2H4 selectivity. We observed 83.2% C2H6 conversion, 76.7% C2H4 selectivity, and 63.8% C2H4 yield over La1.6Sr0.4CuO3.857F0.143> and 79.6% C2H6 conversion, 74.6% C2H4 selectivity, and 59.4% C2H4 yield over La1.6Sr0.4CuO3.856Cl0.126 under the reaction conditions of C2H6/O2/N2 molar ratio 2/1/3.7, temperature 660°C, and space velocity 6000 ml h-1 g-1. With the rise in space velocity, C2H6 conversion decreased, whereas C2H4 selectivity increased. Life studies showed that the two catalysts were durable within 60 h of on‐stream ODE reaction. Based on the results of X‐ray photoelectron spectroscopy, O2 temperature-programmed desorption, and C2H6 and C2H6/O2/N2 (2/1/3.7 molar ratio) pulse studies, we conclude that (i) the inclusion of halide ions in the La1.6Sr0.4CuO4δ lattice could promote lattice oxygen mobility, and (ii) the O- species accommodated in oxygen vacancies and desorbed below 600°C favor ethane complete oxidation whereas the lattice oxygen species desorbed in the 600–700°C range are active for ethane selective oxidation to ethene. By regulating the oxygen vacancy density and Cu3/Cu ratio in the K2NiF4-type halo-oxide catalyst, one can generate a durable catalyst with good performance for the ODE reaction.

Perovskite-type chioro-oxide SrCoO3-δClσ: A novel and durable catalyst for the selective oxidation of ethane to ethene

The catalytic performances and characterization of SrCoO 3-0.401 and SrCoO 3-0.450 Cl 0.126 catalysts have been investigated for the oxidative dehydrogenation of ethane (ODE) to ethene. XRD results indicated that both catalysts were of cubic perovskite-type structures. The incorporation of chloride ions in the SrCoO 3-δ lattice can significantly enhance C 2 H 6 conversion and C 2 H 4 selectivity. The catalyst showed 89.6% C 2 H 6 conversion, 67.1% C 2 H 4 selectivity, and 60.1% C2H4 yield under the reaction conditions of C2H6/O2/N2 = 2/1/3.7, temperature = 660°C, and space velocity = 6000 mL h -1 g -1 . Over SrCoO 3-0.450 Cl 0.126 , with the rise in space velocity, C 2 H 6 conversion decreased, whereas C 2 H 4 selectivity increased. Life studies showed that SrCoO 3-0.450 Cl 0.126 was durable. The results of XPS and pulsing studies indicated that the inclusion of chloride ions in the SrCoO 3-δ lattice could promote lattice oxygen mobility. Based on the results of O 2- TPD and TPR studies, we suggest that the oxygen species desorbed at ca 695°C were active for the selective oxidation of ethane. By regulating the oxygen vacancy density and Co 4+ /Co ratio in the perovskite-type chloro-oxide catalyst, one can generate a durable catalyst with excellent performance for the ODE reaction.